RE: 26 GT Problem
Nate,
The damage to the back-plate is superficial, but it does prove the crankshaft is not properly placed...
Since your engine is new, I would suggest sending it back to Horizon, even though you can probably overcome this issue by yourself.
This is covered by warranty, while any damage done to engine parts while you disassemble/reassemble it will not be.
You will need to fully disassemble this engine, to make sure everything is correctly assembled, including the removal of the rear bearing.
I suspect it is this bearing's improper seating that causes this rubbing; and if you look at the side of the retainer, diametrically opposite where your arrow point; it shows rub-marks as well.
The front bearing is what axially locates the crankshaft and it is, by design, seated as far back as possible in its seat at the front of the crankcase.
The inner race of the front bearing must be held very firmly between the crankshaft's 'step' and the prop-driver.
The only thing that would allow the crankshaft to slide rearward, is if the prop-driver, which is located on the crankshaft by a Woodruff key, is somehow prevented from moving all the way back against the front bearing's inner race.
This can happen if the rear bearing is held away from its seat, or the crankshaft is very firmly kept off from fully mating with this bearing's inner race...
But this would have happened immediately (not after using half a gallon...).
Once the bearing is seated, it will not pop out... There is no reason for it to.